Description

• Bile acid malabsorption occurs when increased primary bile acids in the colon lead to chronic diarrhea and abdominal symptoms such as discomfort and bloating.
• Definitions (1)
  • Type 1: Secondary to ileal dysfunction (e.g., Crohn disease, ileal resection, or radiation injury)
  • Type 2: Idiopathic
  • Type 3: Secondary to GI disorders other than ileal dysfunction (e.g., chronic pancreatitis, postcholecystectomy, celiac disease, bacterial overgrowth, and others)
  • Type 4: Increased bile acid synthesis induced by metformin (does not cause malabsorption but can cause similar symptoms to bile acid malabsorption due to effects of increased bile acid levels in colon)

Epidemiology

Prevalence

• Overall prevalence is estimated to be approximately 1% in Western countries (1).
• Among patients with Crohn disease, with ileal resection, prevalence may be as high as 90% (1).
• Among patients with functional diarrhea or diarrhea-predominant irritable bowel syndrome (IBS-D), ~30–40% shown to have dysregulation bile acid synthesis (1)
• Bile acid malabsorption has been observed in up to 43% of patients with microscopic colitis, although data is conflicting and this was not replicated in other studies (1).
• Prevalence of type 3 and type 4 bile acid malabsorption unknown (1)

Etiology and Pathophysiology

• Primary bile acids are synthesized from cholesterol in the liver and secreted in bile after storage in the gallbladder, where they emulsify fats and play a role in micelle formation (1),(2),(3).
• ~95% of bile acids are actively absorbed in the terminal ileum via the apical sodium bile acid transporter (2).
• In the colon, secondary bile acids are formed by modification of primary bile acids by intestinal bacteria; these modifications increase passive absorption of secondary bile acids in the colon. Ultimately, a small amount of bile acids is passively reabsorbed in the colon (3).
• After reabsorption in the ileum or colon, primary and secondary bile acids return to the liver via the portal vein for reuse (2).
• Bile acid malabsorption in the ileum leads to increased levels of bile acids in the colon. There, the bile acids cause diarrhea via multiple mechanisms: stimulation of water, electrolyte, and mucus secretion; stimulation of colonic motility; alterations to microbiome (1).

Genetics
Genetic variants in proteins involved in enterohepatic circulation of bile acids may contribute to bile acid malabsorption in murine models

Risk Factors

• Inflammatory bowel disease (IBD), primarily Crohn disease (ileal resection or ileal disease)
• Radiation therapy associated ileitis (e.g., pelvic radiation therapy)
• Cholecystectomy
• Microscopic colitis

General Prevention

No specific measures for prevention

Commonly Associated Conditions

• Crohn disease
• Celiac disease
• IBS-D
• Functional diarrhea